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Estimation of mass transfer rate and primary drying times during freeze-drying of frozen maltodextrin solutions based on x-ray μ-computed tomography measurements of pore size distributions

Foerst, Petra, Melo de Carvalho, Teresa, Lechner, Martin, Kovacevic, Tijana, Kim, Seongshik, Kirse, Christoph, Briesen, Heiko
Journal of food engineering 2019 v.260 pp. 50-57
annealing, cakes, cooling, diffusivity, freeze drying, freezing, geometry, maltodextrins, mass transfer, micro-computed tomography, porosity
The study presents a method to estimate the mass transfer rate during primary drying based on μ-computed tomographic measurements of a freeze-dried cake. The study was carried out in a defined setup in a lyomicroscope and confined geometry in which freezing occurred under homogeneous nucleation, and drying under mass-transfer-limited conditions. Pore size distribution, effective diffusivity, and primary drying rates of maltodextrin solutions at different weight concentrations (5%, 10%, and 20% (w/w)) and different cooling rates were determined. Furthermore, the effect of annealing treatment on primary drying rate was studied. Two different annealing treatments were tested, one for 90 min at −10 °C and another for 180 min at −5 °C. It was shown that solid concentration and annealing above Tg’ have the strongest impact on pore size and, subsequently, on primary drying rate. Therefore annealing represents a powerful tool to accelerate primary drying which is the most time-consuming step during lyophilisation.